1. Field of the Invention
The present invention is directed to a circuit for the continuous direct or indirect variation of the direct and/or alternating current flowing through a load operated by a mains DC or AC voltage or an arbitrary combination thereof, of the type having a network that makes an input voltage U.sub.in having an arbitrary shape and frequency f.sub.network available, and with the load being connected to the network via the circuit.
2. Description of the Prior Art
Such loads or loads can, for example, be heating elements formed by ohmic resistors or heating tubes and that exhibit a more or less linear curve of the temperature-resistance characteristic curve. The circuit is also employable for loads having an inductive characteristic such as, for example, motors, transformers or filter circuits capacitive loads such as electrical or electronic circuits or capacitive circuits of other load types as well as filter circuits having capacitive characteristics can likewise be utilized. The loads or loads can have portions composed of different ones of these load types.
Various methods are utilized for regulating the power output to electrical or electronic loads. The goal of these methods is to vary the power supplies to a load or a load in arbitrarily fine steps in a range between 0 and 100% of the nominal power.
The phase controller utilized in alternating current networks represents such a method. In a phase controller, the two half-waves of the mains (public utility) voltage are partially blanked and the power effectively supplied to the load is thus effectively reduced. The power output to the load can be varied between 0 and 100% of the nominal power with the phase controller by varying the phase angle. A disadvantage of this method is that the mains voltage and the mains current differ in terms of their form, i.e. the mains current contains harmonics not present in the mains voltage. These harmonics can negatively influence other loads and lead to considerable problems in the network. These harmonics can be reduced by employing extremely large filters, but this leads to wattless currents and reduction of the effective parts.
Another such method is full-wave or half-wave packet control, that is employed in alternating current networks. In this method, a number of mains full-waves or half-waves are blanked. This means that the load is periodically connected to and disconnected from the network. The power supplies on average to the load is varied in the range between 0 and 100% of nominal power by modifying the on duration. This method has the disadvantage that the load periodically loads the network. Given a non-linear load, this effect is additionally intensified by the increased start current. The periodic network loading generates fluctuations of the mains voltage due to the network impedance which has a disturbing effect, for example in the form of flickering given illumination devices.
Both of the above-described methods for regulating the power supplied to a load can no longer be employed in public networks, or can only be employed therein with additional outlay, to which standards for limiting harmonics (EN61000-3-2 or EN61000-3-4) and for limiting flickering (EN61000-3-3) apply. According to the current status of the discussions in the appertaining European committees, these standards will take effect on Jan. 1, 2001.